The first thing that I probably have to say about this book is that it is effectively the Chronicles of Narnia for adults. While the main character is human, when he is transported to the world of Spellsinger, he is transported to a world inhabited by talking animals. Okay, not all of Lewis' characters in Narnia were talking animals, but a lot of them where. However, Narnia was originally written for children (not to say that adults don't enjoy them) while it is very clear, especially from the antics of Mudge the Otter, that this series is not.
The book is about how a pre-Law student at UCLA, Jon-Tom, is transported to the other world because the turtle wizard Clothahump is looking for a powerful magic user from our world, and believes that he needs the services of an engineer. Jon-Tom is an engineer, a sanitation engineer (that is a cleaner) however when he arrives in the world, he suddenly discovers that he is much more than a simple cleaner. When he gets his hands on an instrument (he is also in a garage band) it turns out that we he uses it magical things happen. What I mostly enjoyed about this series is the fact that when he played the instrument, he would play familiar rock songs from our world, with the resulting consequences.
Alan Dean Foster is probably more known for his novelisations of a number of movies. Actually, to me he is not, he is known to me for the Spellsinger series. These are the only books of his that I have read, and are probably the only books of his that I will read. I have no real interest in reading a novelisation of a film. I find most films that try to base themselves on books to be severely lacking, however I find that novels that are based on films are even more lacking. I generally do not like spin off series either (I do not believe I have ever read a Star Wars book).
This series is quite amusing, and in a way I related to Jon-Tom. There is a mix between him being a pre-Law student at college, working as a janitor so that he may pay his way through (though when I read this series I was still a teenager), but also being a struggling musician that suddenly comes good when he hits his big break, which is not so much in this world, but rather in the world ruled by talking animals.
Apparently there are a few continuity problems with this series, however it has been such a long time since I read them that they really don't stick in my mind. Further, I was enthralled with the character of Jon-Tom, and pretty much read the six books that had been written at the time. Foster has now written another two, however I have moved on from pulp fantasy (which is basically what this series is) so I really have no interested in returning to them.